1. Field of the Invention
The present invention relates to a catalyst for the purification of exhaust gases and, more particularly, to a heat-resistant catalyst for purifying exhaust gases.
2. Description of Related Art
Exhaust gases, such as NOx and CO, from vehicles, such automobiles, are removed, for example, by ternary catalysts which may be disposed in the exhaust system.
U.S. Pat. No. 4,888,320 discloses a catalyst composed of a catalyst carrier with two coats deposited thereon. Specifically, a first coat is deposited on a surface of the catalyst carrier and a second coat is further deposited on a surface of the first coat and, more specifically, the first coat is comprised of alumina and the second coat is comprised of alumina containing cerium oxide CeO.sub.2, platinum Pt and rhodium Rh. As the alumina layer as the first coat is formed under the second coat, the second coat containing a catalyst constituent such as platinum can be made uniform in coat thickness, preventing the catalyst component from sintering.
Japanese Patent Unexamined Publication (kokai) No. 113,487/1975 discloses a catalyst for the purification of exhaust gases in which a platinum group metal is deposited on the coat layer formed on the catalyst carrier comprised of active alumina by preparing a slurry solution containing at least one of chromium and tungsten and at least one of calcium, strontium, barium, silicon, tin and zirconium and coating the slurry solution on the surface of the carrier.
Japanese Patent Unexamined Publication (kokai) No. 71,536/1987 discloses a catalyst consisting of a first coat layer comprised of alumina containing platinum and rhodium and a second coat layer, which is formed on the first coat layer, containing cerium oxide and palladium.
For conventional catalysts for purifying exhaust gases, active alumina used as the catalyst carrier has the tendency that it causes crystal growth as a result of an interaction of the active alumina with the platinum group metal due to heat of the exhaust gases in a reducing atmosphere, thereby resulting in stabilization and heat deterioration. Hence, such catalysts cannot stabilize its activity at low temperatures.